Primary lung tumors in dogs, or tumors that arise directly from the lung tissue, (as opposed to metastatic tumors that have spread to the lung from a primary tumor starting elsewhere in the body), are relatively rare compared to primary lung tumors in humans. The vast majority are carcinomas, of which squamous cell carcinoma is one particular subtype. Squamous cell carcinomas can also arise elsewhere in the body, such as in the oral cavity, skin, and nail beds, and then spread to the lungs. When a tumor mass is noted in the lungs, it is important to determine if this mass is originating in the lung, or if it represents spread from cancer elsewhere in the body. Sometimes the appearance and location of a mass can help in this determination, but a biopsy sample of the tumor tissue is the only way to know for certain.

Primary lung tumors typically occur in older, middle-sized dogs. There is some suggestion that environmental carcinogens (such as an urban environment or second-hand smoke) might play a role in the development of these tumors.

Traditionally, chest radiographs (X-rays) have been used to determine the extent of the cancer. Newer imaging methods such as CT scan or MRI are potentially much more useful. The important factors are size of the primary tumor, whether there is a solitary lesion or multiple masses, whether there is evidence of lymph node involvement, and whether there is fluid accumulation in the chest cavity. If there is a solitary lung mass with no evidence of spread, surgery is the treatment of choice. If there is evidence of spread, there is little in the veterinary literature to evaluate the effectiveness of either chemotherapy or radiation therapy.